US 7301002 B1
Improved sulfonated polyphenylene compositions, improved polymer electrolyte membranes and nanocomposites formed there from for use in fuel cells are described herein. The improved compositions, membranes and nanocomposites formed there from overcome limitations of Nafion® membranes.
1. A sulfonated polyphenylene polymer, derived by controllably sulfonating a polyphenylene polymer with a sulfonating agent, the sulfonated polyphenylene polymer having repeat units of the following structure 1:
R1, R2 and R3 are the same or different, wherein each R1, R2, and R3 is H or an unsubstituted or inertly-substituted aromatic moiety;
Ar1 and Ar2 are the same or different, wherein each Ar1 and Ar2 is an unsubstituted aromatic moiety or inertly-substituted aromatic moiety;
wherein a pendant side chain of a sulfonyl group attaches to a carbon atom;
wherein from one to six sulfonyl groups are attached per repeat unit;
wherein n≦2; and
wherein any combination of R1, R2 and R3 and Ar1 and Ar2 comprises a sub-combination selected from the group consisting of:
a) R1 is different than R2,
b) R1 is different than R3,
c) R2 is different than R3,
d) Ar1 is different than Ar2, and
2. The sulfonated polyphenylene polymer of
3. The sulfonated polyphenylene polymer of
4. The sulfonated polyphenylene polymer of
5. The sulfonated polyphenylene polymer of
6. The sulfonated polyphenylene polymer of
7. The sulfonated polyphenylene polymer of
8. The sulfonated polyphenylene polymer of
where each Z is selected from —O—, —S—, alkylene, —CF2—, —CH2—, —O—CF2—, perfluoroalkyl, perfluoroalkoxy,
where R5 is selected from —H, —CH3, —CH2CH3, —(CH2)CH3, or Ph; and
wherein x=CH3, CEt3, CMe3, CF3, NMe2, NH2, F, Cl, Br, OCH3, OH, OCF3, O-Ph, Ph, and SO3R5.
This application claims benefit under 35 U.S.C. § 119(e) of provisional application No. 60/510,930 filed Oct. 14, 2003, which is incorporated by reference herein, in its entirety, for all purposes.
This invention was made with government support under Contract No. DE-AC04-94AL85000 awarded by the United States Department of Energy's National Nuclear Security Administration. The government has certain rights in the invention.
The present invention relates to sulfonated polymer compositions which are suitable in particular for producing polymer electrolyte membranes, electrodes and membrane electrode assemblies for use in fuel cells, in high-performance capacitors, in dialysis equipment and in ultrafiltration and methods of synthesizing polymer compositions. More specifically, the present invention relates to sulfonated polyphenylene polymers, methods of making the same and the use of a sulfonated polyphenylene polymer composition as a polymer electrolyte membrane in hydrogen fuel cells and direct methanol fuel cells and in electrode casting solutions and electrodes.
Polymer electrolyte fuel cells (PEFCs) have great potential as an environmentally friendly energy source. Fuel cells are electrochemical energy converters which feature in particular a high level of efficiency. Among the various types of fuel cells, PEFCs feature high power density and a low weight to power ratio. The PEFC uses as its electrolyte a polymer membrane.
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Fuel cells are attractive electrical power sources, due to their higher energy efficiency and environmental compatibility compared to the internal combustion engine. The most well-known fuel cells are those using a gaseous fuel (such as hydrogen) with a gaseous oxidant (usually pure oxygen or atmospheric oxygen), and those fuel cells using direct feed organic fuels such as methanol.
The polymer electrolyte membrane or proton exchange membrane (PEM) is an important aspect of any PEFC. PEMs are an excellent conductor of hydrogen ions. The most widely used materials to date consist of a fluorocarbon polymer backbone, similar to Teflon®, to which are attached sulfonic acid groups. The acid molecules are fixed to the polymer and cannot “leak” out, but the protons on these acid groups are free to migrate through the membrane. With the solid polymer electrolyte, electrolyte loss is not an issue with regard to stack life. The potential power generated by a fuel cell stack depends on the number and size of the individual fuel cells that comprise the stack and the surface area of the PEM.
In many fuel cells, the anode and/or cathode comprise a layer of electrically conductive, catalytically active particles (usually in a polymeric binder). A polymer electrolyte membrane is sandwiched between an anode and cathode, and the three components are sealed together to produce a single membrane electrode assembly (MEA). The anode and cathode are prepared by applying a small amount of a catalyst for example platinum (Pt) or ruthenium-platinum (Ru/Pt) in a polymeric binding to a surface that will be in contact with the PEM. Preparation of catalyst electrodes has traditionally been achieved by preparing an ink consisting of an electrocatalyst (either Pt or Ru/Pt), Nafion® polymer (5% wt. solution dispersed in lower alcohol). The ink is applied to porous carbon paper using a painting technique or directly depositing the ink upon the membrane surface or pressing it upon the membrane like a decal.
A MEA of a hydrogen fuel cell typically accepts hydrogen from a fuel gas stream that is consumed at the anode, yielding electrons to the anode and producing hydrogen ions which enter the electrolyte. The polymer electrolyte membrane allows only the hydrogen ions to pass through it to the cathode while the electrons must travel along an external circuit to the cathode thereby creating an electrical current. At the cathode, oxygen combined with electrons from the cathode and hydrogen ions from the electrolyte to produce water. The water does not dissolve in the electrolyte and is, instead, rejected from the back of the cathode into the oxidant gas stream.
For the last 30 years the industry standard for the PEM component of a hydrogen or methanol fuel cell has been membranes based on fluorine-containing polymers, for example the Nafion® material marketed by DuPont.
Nafion material is a perfluorinated sulfonic acid polymers having the following structure
The Nafion® membranes display adequate proton conductivity, chemical resistance, and mechanical strength. Some of the membranes disadvantages are reduced conductivity at high temperatures (>80° C.), high methanol permeability in direct methanol fuel cells, relatively thick membranes and membrane dehydration at high elevated temperatures. Further, when Nafion® membranes are used at temperatures above 80° C., they thermally deform. This deformation of the membrane prevents the Nafion® membrane from coming into sufficient contact with the electrode, thereby reducing fuel cell performance. Additionally, there is a need to reduce the costs associated with such membranes.
Another limitation of Nafion® membranes occurs in applications in methanol fuel cells. Nafion® membranes are permeable to methanol. Methanol crossover is inversely proportional to membrane thickness. Direct transport of the fuel (i.e. methanol) across the membrane to the cathode results in losses in efficiency.
Increasing the membrane thickness results in decreased methanol crossover. However, thicker membranes result in Ohmic losses and decreased fuel cell performance.
Membranes that decrease the rate of methanol crossover would allow the use of higher concentrations of methanol-water feed mixtures, which would increase catalyst efficiency, direct methanol fuel cell power out put and potentially fuel utilization.
In general, increasing the operation temperature of fuel cells is advantageous for several reasons. Higher operating temperatures in methanol fuel cells decrease the carbon monoxide poisoning of the electrocatalyst. Higher temperatures increase reaction kinetics of hydrogen oxidation on the anode and oxygen reduction on the cathode. However, as the temperature is increased, it becomes more difficult to keep the membrane hydrated. Dehydration of membranes is exacerbated by relatively thick membranes. Dehydrated membranes lose ionic conductivity and result in poor contact between fuel cell components due to shrinkage of the membrane.
Therefore, improved performance of fuel cells would be achieved by reducing the thickness of membranes and improving the humidification state of solid PEMS since water molecules can promote proton transport and thin membranes can reduce ionic resistance and Ohmic losses.
Additionally, the contact between the membrane and electrode affects the efficiency of a fuel cell. Interfacial resistance between the membrane and electrode causes Ohmic loss thereby decreasing fuel cell efficiency.
Improving the membrane-electrode contact and continuity wherein the membrane and electrode are cast from a composition having the same or similar polymer electrolytes would improve the membrane-electrode interfacial resistance.
What is needed is a composition and nanocomposites from which improved polymer electrolyte membranes, electrodes and electrode casting solutions are made and that have improved performance at temps at about 80° C. and above. Operating at these temperatures results in enhanced diffusion rates and reaction kinetics for methanol oxidation, oxygen reduction, and CO desorbtion thereby producing a more efficient fuel cell.
Improved compositions, improved polymer electrolyte membranes and nanocomposites formed there from for use in fuel cells, high performance capacitors and dialysis equipment are described herein. The improved compositions, membranes and nanocomposites overcome limitations of Nafion® membranes.
High performance polymer electrolyte membranes and nanocomposites for use in fuel cells are produced from the sulfonated polyphenylene polymer compositions.
One aspect of the present invention provides sulfonated polyphenylene compositions from which high-performance polymer electrolyte membranes are produced.
Yet another aspect of the present invention provides sulfonated polyphenylene compositions from which high-performance sulfonated polyphenylene nanocomposites are produced.
One aspect of the present invention provides for a composition that is useful as a polymer electrolyte membrane of a hydrogen fuel cell.
One aspect of the present invention provides for an electrode casting solution comprising a sulfonated polyphenylene dispersed in a solvent in an amount up to about 20% by weight.
Another aspect of the present invention provides for an electrode for a fuel cell wherein an electrode comprises a sulfonated polyphenylene of the present invention.
Another aspect of the present invention provides for a composition that is useful as a polymer electrolyte membrane of a direct methanol fuel cell.
Still another aspect of the present invention provides for a polymer electrolyte membrane made from sulfonated polyphenylene wherein the sulfonation is inhomogenous.
Still another aspect of the present invention provides for a polymer electrolyte membrane made from sulfonated polyphenylene composition wherein the sulfonation is homogenous.
Yet another aspect of the present invention is a sulfonated polyphenylene polymer with reduced segmental mobility of polymer chains.
Another aspect of the present invention provides for a method of making sulfonated polyphenylene polymer compositions having 2 to 6 pendant sulfonic acid groups per polymer repeat. The ion exchange capacity for sulfonated polyphenylene polymers range from about 0.5 meq (SO3H)/g of polymer to about 5.0 meq (SO3H)/g polymer.
Another aspect of the present invention provides for a polymer electrolyte membrane with improved proton conductance over that of Nafion®117 membranes
Another aspect of the present invention provides for a polymer electrolyte membrane with improved proton conductance over that of Nafion®117 membranes at temperatures in the range of about 90° C. to about 130° C.
Yet another aspect of the present invention provides for a polymer electrolyte membranes that has better mechanical properties than Nafion® 117 membranes at temperatures in the range of about 90° C. to about 130° C.
Another aspect of the present invention provides for a polymer electrolyte membrane with an improved ion exchange capacity over that of Nafion® 117 membranes.
Another aspect of the present invention provides for a polymer electrolyte membrane with a higher temperature of glass transition Tg over that of Nafion® 117.
Still another aspect of the present invention provides for a composition comprising a repeat unit of Structure I.
Yet another aspect of the present invention provides for a polymer electrolyte membrane comprising a polyphenylene polymer wherein the polyphenylene polymer is a sulfonated polymer characterized in that it comprises repeating units having the following structure 1:
Yet another aspect of the present invention provides for an electrode casting solution comprising a sulfonated polyphenylene polymer composition as disclosed herein.
Yet another embodiment of the present invention relates to a membrane electrode assembly comprising an anode; a cathode; and a polymer electrolyte membrane positioned between the anode and cathode wherein at least one of the anode, cathode and polymer electrolyte membrane comprises a sulfonated polyphenylene polymer composition as disclosed herein.
Still another embodiment of the present invention relates to a fuel cell comprising a membrane electrode assembly wherein at least one of the anode, cathode and polymer electrolyte membrane comprises a sulfonated polyphenylene polymer composition as disclosed herein.
Additional objects and advantages of the present invention will be apparent in the following detailed description read in conjunction with the accompanying drawing figures.
Although described herein with respect to preferred embodiments, the present invention is not meant to be so limited, and other modifications and embodiments that fall within the scope of the present invention will be readily apparent to those of skill in the art. The descriptions are illustrative only and should not be viewed as limiting.
For ease of explanation, the present invention is explained in terms of polyphenylene polymer compositions (DAPP) and sulfonated polyphenylene polymer compositions (SDAPP). According to one embodiment of the invention, polyphenylenes are produced through Diels Alder (DA) reactions.
Referring now to
A polymer electrolyte membrane of
The morphology of the parent polyphenylene provides for up to six (6) sulfonic acid moieties to attach to carbons of the aromatic moieties attached to the phenylene backbone of the repeat units (for example when the aromatic moieties are phenyl groups). The number of repeat units in a polymer ranges from 2 or more where n preferably ranges from about 130 to about 300.
Polyphenylene polymers and sulfonated polyphenylene polymers provide for improved thermal stability, while maintaining organic solubility making it possible to form films with improved mechanical properties when compared to Nafion®117 films from Dupont.
According to another embodiment of the present invention a composition comprising polyphenylene repeat units of structure I is polymerized to form polyphenylene polymers of between about 130–3000 repeat units. Structure I is synthesized by various methods including Diels Alder reactions for example where bis-tetraphenylcyclopetadienone reacts with p-Bis(ethynyl)benzene to yield carbon monoxide and polyphenylene. Alternatively polyphenylene and carbon dioxide are produced by the condensation of 2-pyrone with p-Bis(ethynyl)benzene.
The repeat unit contains R1, R2 and R3 which are the same or different, wherein each R1, R2 and R3 is H or an unsubstituted or inertly-substituted aromatic moiety. Ar1 represents an unsubstituted or inertly substituted aromatic moiety; Ar2 represents an unsubstituted or inertly substituted aromatic moiety.
For example, synthesis of DAPPs are performed using a modification of the method known in the art wherein to bis-tetracyclone (50.0 g; 72.4 mmol) and 1,4-diethynylbenzene (9.13 g; 72.4 mmol) in a 500 mL Schlenk flash, diphenyl ether (250 mL) is added and the resulting mixture is frozen in an ice bath. The mixture is freeze-thaw degassed (3×) before heating under argon (1 atm) at 180° C. for 24 h. Periodically, carbon monoxide is vented to avoid over-pressurization of the reaction flask. Subsequently, additional diethynylbenzene (0.10 g; 0.8 mmol) is added to the viscous slurry and the mixture is stirred for an additional 12 h at 180° C. The reaction vessel is then cooled to room temperature and its contents are diluted with toluene (300 mL). The polymer is precipitated by dropwise addition of the solution to 1000 mL of acetone. This dilution in toluene and precipitation in acetone is repeated and the resultant white solid is dried in a vacuum oven for 12 h at 80° C., 48 h at 230° C., and 24 h at room temperature. A 96% yield (52 g collected) of a tough, yellow solid is obtained. According to one embodiment of the present invention, the DAPP polymer is polymerized in the absence of a metal catalyst.
In one example of a sulfonation, DAPP (7.02 g) is added to a flame dried, 500 mL three-neck, round-bottom flask under argon and dissolved in methylene chloride (6% solution by weight). The solution is cooled to −50° C. (dry ice/acetonitrile) and chlorosulfonic acid (4.30 g, 36.9 mmol) (diluted in chloroform 20 mL) is added dropwise though an addition funnel over 15 minutes while being vigorously stirred with a mechanical stirrer under an argon atmosphere. This amount of chlorosulfonic acid gives a 4:1 ratio of acid to polymer repeat unit. Other ratios of sulfonating agent to polymer repeat unit are prepared to attain polymers with various ion exchange capacities. A mixture darkened from bright yellow to black during chlorosulfonic acid addition. After 30 minutes the reaction is warmed to room temperature at which point a dark solid precipitated. The organic solvent was decanted and to the remaining solid was added 300 mL of a 0.5 M solution of NaOH that was allowed to react at room temperature for 12 h. The slurry is then heated for 4 h at 80° C. to ensure sulfonyl chloride conversion. The off-white solid was Soxhlet extracted with de-ionized water for 48 h and dried in a vacuum oven at 100° C. for 48 h to obtain light yellow solid (9.52 g). 1H NMR (D-DMSO): δ(ppm)=broad signal 6.35–7.22 (peaks at 7.19, 7.09, 6.95, 6.88, 6.64, 6.54). 13C NMR (DMSO-d6): δ (ppm)=broad signal 138–141 (peak at 139) broad signal 131–126 (peaks at 126.9, 128.7, 129.5, 131.0).
The sodium salt of the sulfonated polymer is dissolved in DMAc (10% weight solution) and filtered though a 2 μm syringe filter (glass microfiber filter). The solution is cast onto a clean glass plate in an oven at 90° C. under N2. After 20 h, the film is removed from the glass plate and immersed in de-ionized water (1 L, 18 M•) for one hour at 100° C. to extract any residual DMAc. The resultant film is then converted into the acid (proton) form by immersion in 2.0 M H2SO4 at 100° C. for one hour. The film is then rinsed thoroughly with de-ionized water and then soaked in deionized water (18 M•) at 100° C. for another hour to remove any residual acid. The films are stored in de-ionized water until used.
According to one embodiment of the present invention, a 6 wt % polymer solution of DAPP in methylene chloride is homogeneously sulfonated in situ with chlorosulfonic acid. Sulfonating agents include sulfuric acid, fuming sulfuric acid and sulfur trioxide, or a mixture of sulfuric acid and thionyl chloride but are not limited thereto.
Activation of the polyphenylene(s) for proton conductivity is accomplished by using a variety of activating agents. The degree of sulfonation and the homogeneity of sulfonation between repeat units are controlled by varying the concentration of the sulfonating agent, varying the temperature of the reaction and whether a polyphenylene polymer is dissolved at the time of sulfonation. Pendant side chains of sulfonyl groups are attached to a carbon atom of the lateral phenyl ring. According to one embodiment of the present invention, the attachment of the pendant groups are at the para position from attachment to the backbone. According to yet another embodiment of the present invention, the attachment of the pendant groups are at the meta position from attachment to the backbone.
The following examples provided are intended for illustrative purposes and do not limit the scope of the invention since other suitable sulfonating agents will be apparent to one of ordinary skill in the art.
One method for sulfonating a polyphenyl backbone with the illustrative parent polyphenylene structure is illustrated in scheme 1
Polyphenylenes that are not 100% sulfonated are soluble in a variety of polar, aprotic solvents such as as N,N-Dimethylacetamide (DMAC), dimethylformamide (DMF), dimethylsulfoxide (DMSO) and N-Methylpyrrolidone (NMP). Fully sulfonated polyphenylenes are at least partially insoluble in a variety of polar, aprotic solvents such as DMAC, DMF, DMSO and NMP and as such can form a hydrogel. The sulfonated polyphenylenes hydrogels are useful as ion exchange resins for desalination of water and chelation of inorganic materials that would exchange into the hydrogel.
The degree of sulfonation per repeat polymer unit is controlled by the concentration of sulfonating agent. Manipulation of temperature and concentration of the sulfonating agent yields sulfonated polyphenylene polymer compositions having morphology that allows for increased ion exchange capacity, membrane hydration, improved physical properties at temperatures above 80° C. and limited methanol (MeOH) fuel crossover when compared to Nafion® compositions.
A DAPP and SDAPP backbone structure provides decreased segmental mobility thereby producing a relatively stiff backbone structure or rod-like structure. The sulfonic acid pendant groups are distributed throughout the polymer. The overall morphology of the SDAPP provides for a rod-like structure wherein the acid groups form proton channels. The rod-like structure of the DAPP and SDAPP contributes to the improved water retention properties of the materials, the decreased low-carbon fuel permeability and the improved conductance thereby leading to increased fuel cell efficiency over that of Nafion®117 materials.
According to one embodiment of the present invention films are cast upon a clean glass surface from about a 20 wt % solution of polymer. The surface of the glass can be coated with hydrophobic groups. The resulting film is transparent and flexible.
According to another embodiment of the present invention films are treated with H2SO4 for about 24 hours. The treated films are bathed in a bath of deionized water for about 24 hours to remove any inorganic salts. The treated films are further treated to H2SO4 at about 100° C. for about one hour. Inorganic salts deposited on the films are removed by bathing the films in deionized water at about 100° C. for about 1 hour. In an alternative embodiment the second wash step is omitted.
Electrochemical Impedance Spectroscopy (EIS) was used to collect proton conductivity data for polyphenylene polymer materials and sulfonated polyphenylene polymer material. Films are measured by four probe EIS using a Solartron 1260 frequency analyzer and a Solartron 1287 potentiostat and a test cell designed to measure the materials impedance by the point method. The resistance of each film was measured while totally submerged in deionized-water during each measurement at 25° C. Proton conductivity was calculated using Eq. 1
Sulfonated polyphenylene polymer materials are analyzed by infrared spectroscopy. Referring now to
The ion exchange capacity of membranes increase with an increase in the sulfonic acid content of each polymer unit. Referring now to
Shown in Table I, a comparison of proton conductivity, ion exchange capacity (IEC) and water uptake for sulfonated polyphenylene (SDAPP) materials SDAPP1, SDAPP2, SDAPP3, and SDAPP4 is illustrated.
As the sulfonation level increases the ion exchange capacity also increases since the ion exchange capacity is proportional to the sulfonic acid pendant groups attached to the lateral phenyl groups of polyphenylene backbone structure of the polymer. The water uptake (weight %) increases with increased conductivity and ion exchange capacity. Water uptake is measured by immersing the acidified films in deionized water for about 24 hours at about 25° C. The films are them blotted dry and weighted (Ws). The films were then dried and weighted (Wd). Water uptake=[Ws−Wd/Wd]×100%. The films displayed proton conductivities ranging from 8–115 mS/cm while still maintaining mechanical stability.
Thermogravametric analysis of DAPP is compared to the thermogravametric analysis of SDAPP1, SDAPP2, SDAPP3, and SDAPP4 which is illustrated in
Referring now to
Referring now to
Referring now to
Referring now to
Referring now to
Preparation of catalyst electrodes has traditionally been achieved by preparing an ink or dispersion consisting of electrocatalyst (either Pt or Pt/Ru), and Nafion® (5% wt. solution dispersed in lower alcohol) and other additives which are well known to those in the art such as glycol, or water. The ink is applied to porous carbon support using a hot press technique or applied directly to the membrane surface using a painting technique. Referring now to
SDAPP dispersions for use as electrode inks but not limited to are produced by heating the SDAPP solid (IEC 1.5–5 meq/g) in boiling water for 2 hrs. Isopropanol is added to form an about 5–20 wt % SDAPP water/alcohol casting solution or dispersion. The casting solution may be dispersed by stirring, sonication or heating to form a substantially homogenous polymer solution. The casting solution is suitable for making electrodes with one or more electrocatalyst contained therein.
The sulfonated polyphenylenes of the present invention are also useful as battery separators, electrolytes for electrosynthesis cells, electrolytes for electrolysis cells, electrolytes for gas generating electrochemical systems, as ionic membranes in electrochemical sensors, as electrolytes in electrochemical scrubbers and other purification systems and as electrolytes in primary and secondary batteries.
It will be apparent to those skilled in the art that other types of equipment that record the necessary images may be employed and related statistical and analysis techniques may be used without departing from the scope of the invention as claimed. Further, the embodiments described herein are illustrative only and are not meant as limitations on the claims hereof.